Impact of selection for decreased somatic cell score on productive life and culling for mastitis
H. D. Norman*, R. H. Miller, J. R. Wright; Animal Improvement Programs Laboratory, ARS, USDA, Beltsville, MD, USA

Impact of continued selection for decreased somatic cell score (SCS) was examined to determine if such selection produced cows with difficulty responding to bacteria within the mammary gland as expressed by greater mastitis susceptibility and shorter productive life (PL). Holstein artificial-insemination bulls with a predicted transmitting ability (PTA) for SCS based on ≥35 daughters were grouped by quintile based on PTA SCS. Then 25 cow groups were formed based on sire and maternal grandsire (MGS) quintiles. Cows had birth dates from 1993 through 1999 and calving dates from 1995 through 2005. Cows that changed herds or had unreported lactations for their first 5 parities were excluded as were herds with <5 cows. Data were available from 2,556,402 cows in 27,167 herds. Mean cow PL was 27.1 mo; time opportunity was a restricting factor. Mean PTA SCS was 3.26 for sires and 3.25 for MGS for the cow group with highest sire and MGS quintiles and 2.77 and 2.76 for the group with lowest sire and MGS quintiles. Least squares difference in PL was examined on a within-herd basis with cow birth year in the model. Cows from the lowest sire-MGS quintile group had 3.2 mo longer PL than those from the highest sire-MGS quintile group and were less likely to be culled for mastitis (9 versus 13%) based on reported reason for record termination. Advantage for cows with a sire or MGS with lower PTA SCS was nearly linear across groups within MGS or sire quintile, respectively. Difference in PL between cow groups with highest and lowest sire quintiles for PTA SCS ranged from 2.0 to 2.4 mo; corresponding difference for MGS quintiles ranged from 0.8 to 1.2 mo. Because each month of additional PTA PL is valued at $29 in the current USDA lifetime net merit index, a 200-cow herd from the lowest sire-MGS group for PTA SCS would be worth nearly $6,000 more annually than a 200-cow herd from the highest sire-MGS group without considering any additional income or expense associated directly with SCS. Selection for decreased SCS across generations is expected to produce cows that are less likely to be culled for mastitis.

KEYWORDS
somatic cell score
productive life
mastitis